Usually I would put something like this down to fanboyism but I actually think you are onto something here. Del Toro apparently signed for multiple movies, something like 6 was the number I believe, and I just cant see The Collector being that prominent a character in the MCU, GOTG and then maybe ONE more appearance would be it for TC I think. But Thanos, he could be in 10 MCU movies let alone 6 as he is always coming back to life and thinking up new schemes.

Plus getting someone like BDT to The Collector just seems redundant when someone like Terence Stamp would be a much better fit for the character.

EXACTLY! That was another point I was going to make about Benicio being signed up for multiple films and The Collector doesn't seem like a character that would pop up in other films.

Plus I was watching the GOTG comic con panel again and when Benicio was asked about his character he said "I play a character called The Collector... and he collect things" and to me it was like he was hiding something because once again, you wouldn't waste a talent like Benicio Del Toro on a character like The Collector because it could be anyone.

I believe that Marvel already know who they cast as Thanos and I believe it is Benicio and I think their using The Collector as a 'John Harrison' until they announce that he is playing Thanos.